Antimicrobial Therapies

Question 1

What is Prontosil?

Answer 1

First example of a sulphonamide antibiotic. Is bacteriostatic and synthetic. often used with trimethoprim for a symbiotic effect.  Used to treat UTIs and RTIs, bacteraemia and prophylaxis for HIV+ individuals. Becoming more commonly used due to resistance to other microbials, despite some host toxicity. Only acts on gram positive bacteria. 

Question 2

Why might you give a patient multiple different antibiotics?

Answer 2

They may act on different stages of bacterial growth and give a symbiotic effect e.g. sulfonamides and trimethoprim act on two different stages of bacterial development.

Question 3

How do sulfonamides work?

Answer 3

Inhibition of other metabolic processes:  Sulfonamides interfere with folic acid synthesis by preventing addition of para-aminobenzoic acid (PABA) into the folic acid molecule through competing for the enzyme dihydropteroate synthetase.

Question 4

What are beta-lactams?

Answer 4

Antibiotics, all of whom contain the beta lactam ring.
Interfere with the synthesis of the peptidoglycan component of the bacterial cell wall. Examples include Penicillin and methicillin. Bind to penicillin-binding proteins (PBP) which catalyse a number of steps in the synthesis of peptidoglycan.

Question 5

What is an antibiotic and where do the majority come from?

Answer 5

An antibiotic is an antimicrobial agent produced by a microorganism that kills or inhibits other microorganisms. Most antibiotics in use today are produced by soil-dwelling fungi (Penicillium and Cephalosporium) or bacteria (Streptomyces and Bacillus). However, antibiotics commonly used today encompass a range of natural, semi-synthetic and synthetic chemicals with antimicrobial activity.

Question 6

What is an antimicrobial?

Answer 6

Chemical that selectively kills or inhibits microbes (bacteria, fungi, viruses).

Question 7

What is the difference between bactericidal and bacteriostatic?

Answer 7

Bactericidal – kills bacteria.

Bacteriostatic – stops bacteria growing.

Question 8

What is an antiseptic?

Answer 8

Chemical that kills or inhibits microbes that is usually used topically to prevent infection.

Question 9

What is meant by the minimal inhibitory concentration (MIC)?

Answer 9

The lowest concentration of antibiotic required to inhibit growth. The breakpoint is a clinically achievable concentration.

Question 10

How does antibiotic resistance develop?

Answer 10

Population contains cells with AB resistance due to mutations/acquired DNA – possibly with a fitness cost e.g. Slow growth.
In absence of selection pressure (e.g. ABs) AB resistant strains have no advantage (and may have a disadvantage). Leads to low prevalence of AB resistant strains in patient population
In presence of selection pressure (e.g. Abs) resistant mutants outcompete. High prevalence of AB resistant strains in patient population.

Question 11

What were misconceptions about antibiotic resistance initially?

Answer 11

1. Resistance against more than one class of antibiotics at the same time would not occur.
2. Horizontal gene transfer would not occur.
3. Resistant organisms would be significantly less ‘fit’

Question 12

What are the effects of antibiotic resistance?

Answer 12

1. Increased time to effective therapy.
2. Requirement for additional approaches – e.g. surgery.
3. Use of expensive therapy (newer drugs).
4. Use of more toxic drugs e.g. vancomycin.
5. Use of less effective ‘second choice’ antibiotics.

Question 13

What are the major Gram Negative AB resistant bacterial pathogens?

Answer 13

Pseudomonas aeruginosa
Cystic fibrosis, burn wound infections. Survives on abiotic surfaces.

E. Coli (ESBL) and E. coli, Klebsiella spp (NDM-1)
GI infection, neonatal meningitis, septicaemia, UTI.

Salmonella spp. (MDR)
GI infection , typhoid fever.

Acinetobacter baumannii (MDRAB) 
Opportunistic, wounds, UTI, pneumonia (VAP). Survives on abiotic surfaces. 

Neisseria gonorrhoeae
Gonorrhoea.

Question 14

What are the major Gram Positive AB resistant bacterial pathogens?

Answer 14

Staphylococcus aureus (MRSA, VISA) 
Wound and skin infect. pneumonia, septicaemia, infective endocarditis. 

Streptococcus pneumoniae
Pneumonia, septicaemia.

Clostridium difficle
Pseudomembranous colitis, antibiotic-associated diarrhoea.

Enterococcus spp (VRE) 
UTI, bacteraemia, infective endocarditis. 

Mycobacterium tuberculosis (MDRTB, XDRTB) 
Tuberculosis

Question 15

What are aminoglycosides, how do they work, and when are they used?

Answer 15

Bactericidal antibiotics such as gentamycin or stretomycin. Target protein synthesis (30S ribosomal subunit), RNA proofreading and cause damage to cell membrane. Some toxicity thus limited use - but resistance to other antibiotics has led to increasing use.

Question 16

What is rifampicin?

Answer 16

A bactericidal AB which targets the RpoB subunit of RNA polymerase. Spontaneous resistance is frequent.
It makes secretions go orange/red which affects compliance.

Question 17

What is vancomycin?

Answer 17

A bactericidal AB which targets lipid II component of cell wall biosynthesis as well as wall crosslinking via D-alanine residues. Toxicity means it has limited use, but resistance to other antibiotics has lead to increasing use e.g against MRSA

Question 18

What is linezolid?

Answer 18

A bacteriostatic AB which inhibits the initiation of protein synthesis by binding to the 50S rRNA subunit.
It has a gram-positive spectrum of activity.

Question 19

What is daptomycin?

Answer 19

Bactericidal AB which targets the bacterial cell membrane. It has a gram-positive spectrum of activity.
Its toxicity limits its dose.

Question 20

Why don’t antibiotics harm human cells?

Answer 20

ABs have selective toxicity. They target different bacterial processes not found in mammals.

Question 21

Which different bacterial processes do ABs target?

Answer 21

1. DNA replication
2. Cell wall synthesis
3. Plasma membrane damage
4. Protein synthesis
    - Transcription
   - Translation
5. Enzymatic activity/synthesis of metabolites

Question 22

What are the 4 mechanisms of antibiotic resistance?

Answer 22

1. Altered target site
2. Inactivation of the antibiotic
3. Altered metabolism
4. Decreased drug accumulation

Question 23

How do altered target sites arise in resistant bacteria? Provide examples

Answer 23

Can arise via acquisition of alternative gene or a gene that encodes a target-modifying enzyme.
Methicillin-resistant Staphylococcus aureus (MRSA) encodes an alternative PBP (PBP2a) with low affinity for beta-lactams.
Streptococcus pneumoniae resistance to erythromycin occurs via the acquisition of the erm gene, which encodes an enzyme that methylates the AB target site in the 50S ribosomal subunit.

Question 24

How do resistant bacteria inactivate antibiotics? Provide examples

Answer 24

Enzymatic degradation or alteration, rendering antibiotic ineffective.
Examples include beta-lactamase (bla) and chloramphenicol acetyl-transferase (cat).
ESBL and NDM-1 are examples of broad-spectrum beta-lactamases (can degrade a wide range of beta-lactams, including newest)

Question 25

How does penicillin resistance work?

Answer 25

Penicillin is a beta lactam and so beta lactamases break a bond in the beta lactam ring of penicillin, disabling the molecule.

Question 26

How do bacteria alter their metabolism to resist the effects of antibiotics?

Answer 26

Increased production of enzyme substrate can out-compete antibiotic inhibitor (e.g. increased production of PABA confers resistance to sulfonamides). Alternatively, bacteria switch to other metabolic pathways, reducing requirement for PABA.

Question 27

How do resistant bacteria decrease drug accumulation?

Answer 27

Reduced penetration of AB into bacterial cell (permeability) and/or increased efflux of AB out of the cell – drug does not reach concentration required to be effective

Question 28

What are macrolides?

Answer 28

Antibiotics used to treat Gram-positive and some Gram-negative infections. Targets 50S ribosomal subunit preventing amino-acyl transfer and thus truncation of polypeptides e.g. Erythromycin, azithromycin.

Question 29

What are quinolones?

Answer 29

Synthetic, broad spectrum, bactericidal ABs. Target DNA gyrase in Gm-ve and topoisomerase IV in Gm+ve

Question 30

What are the 3 sources of antibiotic resistance genes?

Answer 30

Plasmids – extra-chromosomal circular DNA, often multiple copy. Often carry mutliple AB resistance genes – selection for one maintains resistance to all. Transposons. Integrate into chromosomal DNA. Allow transfer of genes from plasmid to chromosome and vice versa. Naked DNA. DNA from dead bacteria released into environment.

Question 31

What are the three mechanisms for horizontal spread of AB resistance in bacteria?

Answer 31

Transformation (uptake of extracellular DNA), Conjugation (pilus-mediated DNA transfer) and Transduction (phage-mediated DNA transfer).

Question 32

What are the non-genetic mechanisms for resistance/treatment failure?

Answer 32

1. Biofilm 2. Intracellular location 3. Slow growth 4. Spores 5. Persisters

Question 33

What are the 5 given reasons for treatment failure aside from AB resistance?

Answer 33

1. Inappropriate choice for organism 2. Poor penetration of AB into target site 3. Inappropriate dose (half life) 4. Inappropriate administration (oral vs IV) 5. Presence of AB resistance within commensal flora e.g. secretion of beta-lactamase

Question 34

How can you measure AB resistance?

Answer 34

1. Swabs are typically streaked out onto diagnostic agar to identify causative organism. 2. Once identified, the pathogen streaked over a plate and then over-laid with AB-containing test strips or discs. 3. Other approaches include broth micro-dilution and PCR detection of resistance genes.

Question 35

Why is AB resistance common in hospitals?

Answer 35

Large numbers of infected people receiving high doses of antibiotics - strong selective pressure for emergence/maintenance of AB resistance

Question 36

What are the risk factors for Hospital Acquired Infections?

Answer 36

* High number of ill people! (immunosuppression) * Crowded wards * Presence of pathogens * Broken skin – surgical wound/IV catheter * Indwelling devices - intubation * AB therapy may suppress normal flora * Transmission by staff – contact with multiple patients PINC BIT

Question 37

What are some common pathogens picked up through noscomial infection?

Answer 37

``` Methicillin-resistant S. aureus (MRSA) Vancomycin-insensitive S. aureus (VISA) Clostridium difficle Vancomycin-resistant enterococci (VRE) E. coli (ESBL/NDM-1) P. aeruginosa Acineterbacter baumannii Stenotrophomonas maltophilia ```

Question 38

What harm can come about when antibiotic therapy impairs commensal flora?

Answer 38

In health, commensal organisms can out-compete pathogen with regards to adhesion, metabolism, growth. Pathogen cannot colonise at levels sufficient for infection. After AB therapy => pathogen has no competition which can lead to overgrowth.  When a pathogen then produces toxins which damages the host, it becomes a symptomatic infection, and can spread to other patients

Question 39

How can we prevent the emergence of drug resistant bacteria and nosocomial infections?

Answer 39

1. Prescribing strategies – tighter controls, temporary withdrawal of certain classes. Restriction of ABs for certain serious infections 2. Reduce use of broad-spectrum antibiotics 3. Quicker identification of infections caused by resistant strains 4. Knowledge of local strains/resistance patterns 5. Modification of existing medications to e.g. Prevent cleavage (beta-lactams) or enhance efficacy. E.g. Methicillin. 6. Combinations of antibiotic + inhibitor of e.g. Beta-lactamase. E.g. Augmentin.

Question 40

What are fungi?

Answer 40

Fungi are eukaryote organisms that digest their food outside of the cell by secreting hydrolytic enzymes which can break down biopolymers to be absorbed for nutrition.

Question 41

What classes of conditions do fungi cause?

Answer 41

1. Allergy – allergic reactions to fungal products e.g. allergic bronchopulmonary aspergillosis (ABPA) 2. Mycotoxicoses – ingestion of fungi and their toxic products e. g. aflatoxin 3. Mycoses – superficial, subcutaneous or systemic colonisation, invasion and destruction of human tissue.

Question 42

What are the targets for antifungal therapy?

Answer 42

1. Cell membrane - fungi use principally ergosterol instead of cholestrol 2. DNA synthesis - some compounds may be selectively activated by fungi, arresting DNA synthesis 3. Cell wall - fungi have a cell wall